This invention relates to an improved method for making a novel color cathode ray tube of a rectangular type having a flangeless faceplate and a color selection electrode, which insures that a phosphor screen pattern deposited on the faceplate inner surface is accurately referenced to the source of electron beams for the tube. "Phosphor screen pattern " is herein intended to mean a pattern of interleaved arrays of red-emissive, blue-emissive and green-emissive cathodo-luminescent elements and, in tubes of the negative guardband type, the associated "black grille."
Conventional color cathode ray tubes have a glass envelope which comprises a flanged front panel sealed to a funnel. The front panel flange has embedded in its inner surface a plurality of studs which serve to support a color selection electrode adjacent to a phosphor screen pattern deposited on the inner surface of a faceplate section of the front panel. A neck for housing electron guns for the tube is sealed to the funnel.
It is critical to proper tube operation that the phosphor screen pattern and the aperture pattern of the associated color selection electrode be aligned with respect to the effective source of electron beams in the assembled tube in the same way that they were aligned with respect to the effective point source used in the photo-exposure operations employed to form the screen. If this corresponding relative alignment is not achieved, color purity errors will inevitably be exhibited in the images displayed by the end product tube.
In conventional tubes, the necessary referencing of the phosphor screen and the effective source of electron beams is established by the following method. The conventional referencing method will be best understood if the reader keeps in mind the general principle that two things referenced to a third thing are referenced to each other.
It is conventional during the photoscreening operations in which the phosphor arrays (and the black grille in negative guardband tubes) are deposited, to reference the front panel to three external reference points, typically three rigid posts against which the front panel is urged during the photoscreening operations. Thus, the screen pattern is deposited on the faceplate with reference to three fixed, known reference points. This referencing principle is portrayed in FIG. 1 wherein R represents the fixed external reference (the three reference posts), and reference line RL.sub.a represents the referencing of the screen pattern 2 on faceplate portion 3 of a front panel 4 to the reference R.
During sealing of the neck 5 to the funnel 6 (depicted diagrammatically in FIG. 2) the effective source 7 of electron beams generated by the electron guns (shown schematically as 8), i.e., the apparent "center of deflection" of the beams, is referenced to three corresponding external reference points. More particularly, in conventional practice the neck 5 is sealed to the funnel 6 in a glass-to-glass sealing operation. The center line C/L of the neck 5 (on which the effective source 7 of electron beams will ultimately lie) is then referenced to the said three corresponding external reference points by grinding reference surfaces 9 (commonly termed "pads") on the outside surface of the funnel to the required high degree of accuracy (typically about .+-. 12 mils tolerance). The neck center line 8 and effective source 7 of electron beams are thus referenced to reference R, as represented by reference line RL.sub.b.
To assemble a conventional tube according to conventional practice, a sealing fixture is employed which has in an upper plane three carbon buttons defining three reference points which correspond in location to the afore-described three reference points used in photoscreening the front panel. These panel referencing buttons engage and position the front panel during the funnel-panel sealing operation. In a lower plane, a similar set of three funnel referencing buttons are provided which are aligned with respect to the panel referencing buttons and which are positioned to engage the three external reference surfaces 9 ground on the funnel 6. Such a fixture is described in U.S. Pat. No. 3,737,065.
To seal a conventional funnel and front panel, a frit-type solder glass is deposited on the funnel seal land and the funnel is placed in a fixture, such as described, with the funnel bearing against the said three panel referencing buttons. The front panel is placed on the funnel against the three panel referencing buttons. Since the panel referencing buttons and the funnel referencing buttons are aligned with respect to each other and can be considered as a single reference, it can be understood that the screen pattern on the front panel is thus referenced to the center line of the tube neck and thereby to the effective source of electron beams when the electron gun is ultimately assembled in the tube neck.
The referencing principles which apply to the described panel-funnel sealing operation are shown diagrammatically in FIG. 3. It can be seen that since the screen pattern 2 and neck center line are both referenced to the same external reference R, they are therefore referenced to each other.
The described conventional method for assembling conventional color tube envelopes accomplishes referencing of the phosphor screen pattern to the effective source of electron beams and is satisfactory from a performance and yield standpoint when the sealing fixtures are in good working condition -- yet, the conventional method has a number of drawbacks. Note in FIG. 3 that the external reference R (the six reference buttons on the conventional sealing fixture) must be preserved during the panel-funnel sealing operation. These sealing fixtures are subjected to extreme temperature cycling as they are heated and cooled during the frit sealing operation. The temperature cycling of these fixtures inevitably results in accuracy degradation with resultant high maintenance cost and decrease in yield of the sealing operation due to referencing accuracy losses. Also, the described conventional referencing method requires that high accuracy reference surfaces be ground on the funnel, an operation which adds to the cost of the end product tube.